If you've ever struggled to use your cell phone in your office, Denise Cromwell feels your pain. Her office is in a metal building, nearly impenetrable for wireless signals.

"If it was a call I was expecting and didn't want to miss, I'd literally run out to the parking lot to take it," says Cromwell, vice president of operations at Accurate Elevator & Lift in Middleboro, Mass. "Now I can sit at my desk and take the call."

Cromwell's remedy? A femtocell, which is a signal booster about the size of a Wi-Fi router. Designed to cover no more than a few thousand square feet, femtocells are a new way for wireless service providers to plug coverage holes--with the help of customers like you.

Femtocells plug into your DSL or other wired broadband connection. When your cell phone is within range, the femtocell automatically takes over all incoming and outgoing calls, instead of their going over the outdoor network.

Despite their diminutive size, femtocells can blanket the inside of an office or home with a signal stronger than one from a tower outside, minimizing dropped calls and making the audio clearer.

"You definitely notice a difference in quality," says Cromwell, who uses Sprint's Airave brand of femtocell. Verizon Wireless offers a similar product, called Network Extender; AT&T's version is the 3G MicroCell.

Femtos typically can handle as many as three or four calls simultaneously, but later versions will support 12 or more calls, making them a better fit for larger offices.

Service providers often pair their femtos with a special rate plan that for some users can make wireless calls cheap enough for them to drop one or more of their wired lines. For example, Sprint Airave users pay $10 to

$20 on top of their regular plan to get unlimited calls through the femto, which costs $99. Verizon charges $199 for the hardware but nothing extra for calls.

For customers who spend a lot of money and still complain about spotty service, some carriers will give away the femto or the calling plan--but rarely both.

"People just accept that in some areas, you can't get service," says Cromwell, who also has a femto in her home and in her service manager's home so that clients can always reach them. "That's not the case anymore."